Heat exchanger tubing manufacture



Aug 20, 1963 A. c. K. NIHLEN ETAL 3,100,930

INVENTORS jqwidamfzzumw Fay 8 Y Robermwww a 5km@ 421@ War M16; ATTORNEYS i Aug. 20, 1963 HEAT ExcHANGER TUBING MANUFACTURE original Filed Dec. so, 1959 V 1 4 Sheets-Sheet 3 JNVENToRs rvz'al CKNb'lzZen q, F 13 BY RaberMWaZZaae ATTORNEYS A. c. 4K. NlHLi-:N ETAL 3,100,930

Allg' 20, 1963 A. c. K. NxHLl-:N ETAL 3,100,930

HEAT EXCHANGER TUBING MANUFACTURE so, 195s 4VSheetS-Sheet 4 Original Filed Dec.

ATTORNEYS might be done is to extrude l ing directly over the steel tubing and thereby/provide the` proper heat 'flow bei` United States Patent O Uur invention relates to heat exchanger tubing rmanufacture, and more specifically to methods for forming internal and external sheathed heat exchanger tubing, both linned and uniinned. Even more specifically, our invention relates to methods for forming sheathedheat exchanger tubing in whichthe tubing is formed with a sheathing covering substantially the entire inner surface or substantially the entire peripheral surface of the tubing, or both, but in each case is united with the tubing in a physical metal-to-rnetal interlock or intcrengage-ment. Further, this application is a division of our co-pending application Serial No. 862,997, tiled December 30, 1959, now abandoned.

In certain uses of heat exchanger tubing and particularly heat exchanger tubing having fins formed on the outer surface thereof -for use in various forms of heat exchangers of various constructions and for various uses, it has been necessary to use steel tubing in order to withstand the high pressures of the iiuids iiowing through the tubing. This is becoming more and more prevalent in modern heat exchanger design, in view of the higher and higher pressures encountered and the necessity of using the steel tubes in order to gain the strength necessary to withstand these high pressures.

Furthermore, in certain cases where it is absolutely necessary to use the steel tubing in order to withstand the pressures, the fluids within or surrounding the tubes arehighly corrosive to the steel, thereby swiftly corroding and deterioratingthe steel tubing and requiring frequent replacement resulting in high maintenance cost and frequent periods during which'the heat exchanger S Claims.

YCC

tubing and the outer aluminum or copper sheathing will only be maintained sufficient to supply proper heat flow up to a certain maximum temperature, after which the aluminum or copper, expanding at a greater rate, will pull away -from the steel inner tubing, resulting in decreasingly poorer heat transfer as the temperatures increase.

it is, therefore, desirable to provide methods for making an outwardly sheathed tubingV of the foregoing general construction in which an interlock is formed between the inner steel tubing and outer aluminum or copper sheathing which will prevent separation of the sheathing `from. the steel tubing despite the temperatures to which the construction is subjected. Y

One form of prior construction which eliminates certain of the difficulties of the foregoing prior sheathed tubing construction is disclosed in the co-pending U.S. application entitled Finned ri`ubing Manufacture, Serial No. 796,794, filed March 3, 1959, one of' the inventors o which is a co-inventor of the present application. In the construction of this co-pending application, generally L-shaped cross-section or footed fins, for-med of preferably aluminum or copper, are wound on the steel tubing, with the foot portions of the fins abutting to pro- F vide an unbroken sheathing covering the steel tubing.

The foot portions of these fins are then formed into grooves provided in the steel tubing outer surface to thereby interleck the fins with the tubing in a metal-.tometal interengagement which will resist separation between the fins and rtubing at the higher temperatures.

Although this prior footed iin construction provides satisfactory results and is a marked improvement over the prior constructions, `it necessitates the maintaining of proper abutment of the foot portions on the tins with adjacent ns in order to provide the duid-tight sheathing for the steel tubingV and supply the desired corrosion protection. "Thus, relatively close tolerances must be equipmentl cannot be used. Thus, it becomes vital in such constructions to provide some form 'of` protection for the steeltubing, `but yet this protection must be such as to provide efficient heat llow in order that the tubing mayfaccomplish its heat exchanging function.

One such `form of heat exchanger tubing construction having protection from surrounding iluids has been provided which is relatively satisfactory under certain restricted conditions. in this construction, the steel tubing is outwardly sheathed :with a material not subject to the rapid corrosion, such as aluminum or copper, t0 thereby protect the steel tubing from the corrosive fluids. l'lhis prior construction of outwardly sheathed tubing has been formed by telescoping the steel Atubing with aluminum or copper tubing and, in some manner, providing a `tight finterference`iit betweenjthe steel and` the Onemethod by which this aluminum or copper tubing;

aluminum or copper tub-p.

tight tit `required therebetween for tweenthe two materials. i

The principal difficulty with this prior form of out-` wardly sheathed tubing is that the maximum temperature at which this construction `will provide eicient heat transfer is greatly limited in View of the variations between the coefiicients of thermal expansion of the steel and the aluminum or copper. In view of these didierences of coeicients of thermal expansion `between the two materials used, the tight t between the inner steel maintained in the lin windingoperation, and close inspection standards must be observed to be sure thatthe construction is properly formed.

Another prior form of `heat exchanger tubing,in this case providing protection -for the basic steel tubing against internal corrosive liquids, has been formed by telescoping a basic steel tube over a sheathing tube formed Ifrom the aluminum or copper and providing the tight interference fit therebetween. Here again it is merely the tight abutment of the outer surface of the sheathing tube and the inner surface of the basic steel tube which provides the heat flow path through the nal internally sheathed tubing. Y

Such construction is also relatively satisfactory under certain restricted conditions, that is, if the temperature dilerentials encountered are within restricted limits. l the outer basic tubeis formed fof steel and the inner sheathing tube is formed of `aluminum orcopper "-inl this particular `internal-ly,sheathed tubing cons-trutionthe first timetliat this tubing constructionis, heated tez-relatively high temperatures, the inner `sheathing tubefwill tend to force more tightly against theu outer basictube, again,

. in view of the variations vbetween the coeliiclie'ntsof thermal expansion of the steel and the'aluminu'm or copper.

After `thi-s iirst heating, however, it this internally sheathedtubing is then used `at a lower temperature, the inner sheathing tube of aluminum or copper, after having once expanded, will contract inwardly away from the basic steel tube, thereby interrupting the heat flow path ythrough this combined tubing construction. This same condition is present where the internally sheathed tubing is used in a heat exchanger which operates at temperatures lower than that at which the internally sheathed maintaining the basic tube land sheathing tube, each formedl of materials having different coefficients of thermal Vexlpansion, in tight abutment under varying temperature conditions in order to maintain a proper heat ilow path therethrough. v Y l r It is, therefore, a general object of the pre-sent invention yto provide methods for forming a sheathed tubing construction in` which la substantially continuous sheathing is formed internally or externally on the tubing to be protected and is tightly secured to the outer or inner tubing in a manner lsuch that proper heat transfer through the tubing and sheathing will `be maintained at all times l and under greatly varying temperature conditions.

vIt is a primary object of the present invention to provide methods for forming a sheathed tubing construction in which a metal-to-rnetal interengagement or interlock is formed between the `substantially continuous tubular sheathing and outer or inner tubing to maintain the sheathing and tubing in close and proper heat transfer relationship.-

Itis Aa further object of the pres-ent invention to provide methods for forming a sheathed tubing construction in` which Ithe final tubing may be formed finned or unlinned,

as desired, and without altering the basic construction, but rather merely -by including the additional step of adding the tins to the basic construction whether the tubing exposed is the sheathing or the tubing to be protected.

It is still a further object of the present invention to provide methods for forming sheathed tubing in-Which many of the operations of the method may be carried out at the tube mill where the tube is originally formed, and it may be only necessary to add the tins to the tubing, if desired.

It is `also an object of the present invention to provide 4 sheathing tubing formed into Ithe dove-tail grooves to provide the metal-to-metal interengagement or interlock. Finally, the construction may include tins secured to the outer tubing, whether the basic `or sheathing tubing, with these ns being preferably secured in grooves formed in the tubing for such tins.

The methods of the present invention may be stated generally as including the steps of first forming preferably dove-tailcross-section grooves in the inner or outer 'surface lof basic tubing, and these ,groovesV except as limited by internal accessibility may be formed as axially spaced circumferentially extending grooves, circumferentially spaced axially extending grooves, hel-ically extending methods for forming a sheathed tubing construction in v which the metal-to-metal interengagement or interlock between the sheathing and tubing maybe increased or de'- creased in strength asV is required for 'the particular final temperature conditions under which theA sheathed tubing 4will be used.

Finally, it isian object of the present invention to'provideinethodsy for forming sheathed tubing `which satisfies all ofthe above objects yet comprises a minimum of procedural steps and requires a minimum of inspection procednre for maintaining a quality product. i i

These `and other lobjects areaccomplished by the parts, constructions, arrangements, combinations, subcombinations, methods and procedural steps comprising the present invention, the nature of which is set forth in the fol- `lowing generalv statement, preferred embodiments of which-illustrative of the best mode in which applicants have contemplated applying the principles-ar'e set forth in ,thefoljlowing description and illustrated in the accompanying drawings, ,andnwhich are particularly and distinctlyy pointed out and set forth inthe appended claims forming fa part-hereof: Y

ingeneral terms', the sheathed 'tubingV construction grooves, or variations 4and combinations of these forms of grooves as the particular conditions demand. The methlod further may include the stepsv of then telescoping'.

the basic tubing with a second sheathing tubing and forming the second sheathing tubing into the grooves. formed on the adjoining surface of the basic tubing to provide'a metal-tol-'metal interengagement or interlock between the basic and sheathing telescoped tubing. Finally, the method may include-the step of applying tins to the tubing in the outer position preferably in a conventional manner.

By way of example, embodiments of the heat exchanger tubing construction land the steps of certain methods of the present invention are illustrated in the accompanying drawings forming a part hereof, wherein like nu;V merals indicate similar parts throughout the several views, and in which: v

FIG. 1 is a fragmentary radial sectional view of the basic .tube after originally forming circumferentially spaced, axial or longitudinal, generally U-Yshaped grooves inthe outer surface thereof;

FIG. -2, a view similar to FIG. l with the grooves ofV FIG. 1 formed into dove-tail cross section;

' FIG. 3, a view similar to FIG. 2, with an outer sheath-V ing tube telescoped over the'basic tube and prior to por# tions of this outer sheathing Vbeing `formed into the grooves of FIG. 2; Y

FIG. 4, a view similar to FIG. 3, but with the outer sheathing formed into the dove-tail `grooves in finished form providing externally sheathed tubing according to the present invention;

FIG. 5, a fragmentary axial sectional view ofthe externally sheathedtubing of FIG. 4, with helical fins se-Y Y cured to the outer sheathing tube; l

FIG. 6, a fragmentary radial sectional view, showing the forming of the U-shaped grooves of FIG. 1;

FIG. 7, a fragmentary radial sectional view, part in elevation, showing the forming of the dove-tail grooves- FIG. 101,a view similar 5to FIG. 9, but with helical grooves formed in the outer "surface of Ithe basic tube;

fomdby the methods of the presentVv invention may be.'

stated -as includingabasic tubing,`preferably formed of steel, encased in 1an outer tubing or telescoped over an inner sheathing tubing, with this sheathing preferably being formed of a relatively non-corrosive material, such 'as aluminum or copper, yand which sheathing material is preferably softer than the basic tubing material. Furthermore, the construction includes a physical metal-tometal interengagement or interlock betweenV the basic tubing and sheathing tubing, preferably comprised of dovetail cross-section 4groovesfo'rmed 'on the surface of the basic tubing adjacent the sheathing tubing and with the `cir'curnferentially "extending,

FIG. 1,1, a View similar to FIG. 9, but with the combi-KV nation of circumferentiallyspaced, axially extending and:

formed `in' the outer surface of the basic tube;

FIG. l2, a fragmentary sectional view, partiu elevlal A tion and with parts broken away, showing a construction FIG.' 13, a fragmentary sectional view, part in elevation and with parts broken away, looking in the direction of i l the arrows 13--713 in FIGflZ;

axially spaced .grooves spaced, axial or longitudinal, generally dove-tail cross# section grooves in the inner surface thereof;

FIG. 15, la view similar to FIG. 14, with lan inner sheathing tube telescoped within the basic tube and prior to portions of this inner sheathing being formed into the grooves of FIG. 14;

FIG. 16, a yview similar to FIG. l5, but with the inner sheathing formed into the dove-tail `grooves in tinished form, providing internally sheathed tubing according to the present invention; and i FIG. 17,` a fragmentary axial sectional view of the internally sheathed tubing of FIG. 16 with helical tins secured to the outer basic tube.

One embodiment of theV sheathed tubing construction formed by the methods lcf the present invention is shown at various stages of manufacture in FIGS. 1, 2 and 3, i

with the Afinished construction being shown in FIGS. 4 or 5, dependent on whether or not it is desired to provide merelysheathed tubing according to the principles of the present invention or sheathed finned tubing. Further, in this oase ythetubing is externally sheathed tubing.

Referring to the sheathed tubing construction of FIG. 4, an inner basic tube 15, preferably of steel for Vgaining the high strength required, is telescoped within the relatively thin youter sheathingtube 16, with the sheathing tube 16 preferably being formed of a softer relatively noncorrosive material, such 'as aluminum or copper. Fur-` ther, preferably an interference it is provided between the basic tube 15 tand sheathing tube 1o by any usual means such as originally extruding the sheathing tube le over the inner basic tube 15 or by telescoping the two tubes and then rolling the sheathing tube 16 tightly against the inner basic tube 15.

`If the construction of sheathed tubing thus far described We-re used in heat exchanger apparatus, although the interference lit between the sheathing tube 16- and inner basic tube I15 would maintain these tubes tightly together under relatively [low-temperature conditions, in high-tempenature Work, due tothe variations between the coeiiicients of thermal expansion of the materials forming these tubes, these tubes will separate and thereby `interrupt the paths of heat iiow between thetubes resulting in poor heat transfer characteristics for the combined structure. Thus, it is necessary to provide some permanent bond, resistant to the forces created by these variations between the coefcients of thermal expansion of the various materials, and thereby also provide proper heat flow under the high-temperature conditions.

An interengagement or metal-to-metal interlock is, thererfore, provided by the preferably dove-tail ygrooves 17 formed in the outer surface of the basic tube 15 and into which have been formed the sheathing metal portions 18, which portions 1u :are formed integral with the remainder of the `sheathing tube 16. `In this first embodiment construction, the `grooves 17 are circumferentially spaced and extend generally axially or longitudinally ofy the tubes `15 yand 16,.and the. sheathing!metalportions 18 Vare properly formed into the dove-tail contour `of ithesegrooves in order to provideV the securemetahto-metal interlock or interengagement between the :inner basic tube 15 and A further `important factor is thatithe generallyradiallfy extending sides"19 of the sheathing metal portions 118 are tightly abutting Vthe generally radially'extendingside- Walls Ztlof the grooveslin orderto insure apr-oper `path of heat ow between theinner basic tube 15 and sheathing tube 16, even though portions of the sheathing tube might separate froml the inner basic tube-at locations spaced be` tween Ithese `grooves 17 `runder the high-temperature conditions, Finally, a fur-ther preferable limitation `'is that the `combined generally radially extending lengths of the sides 19 of the sheathing metal portions 18 in abutting contact with the side walls 2t? of groove 17 are at least as great as the `generally cimcumerential width of the sheathing metal portions 1S at the mouths of the grooves 17 or at the outer circumferential sur-face of the inner basic tube '15, in order to provide at least an equal path of heat flow between the sheathing metal portions 1S and inner basic tube 15 as is provided between the main portions of the sheathing -tube 16 and the sheathing ymetal por-tions 18.

Where iins are desired on this first embodiment sheathed tubing construction, such iins may be mounted on the outer surface of the sheathed tube 16 in any conventional manner, preferably in grooves formed in this outer surface, and these tins also would preferably be of the same material as the sheathing tube 16, such as the aluminum or copper, to thereby have the same coeflicient of thermal expansion as the material of the sheathing tube. For instance, axially or longitudinally extending ns may be added to the sheathing tube 16 according to the'R. C. Jones et al. patents Nos. 1,921,928 and 1,921,975, or

helical fins may be added to the sheathing tube V1-5 according to the E. A. Dewald Patent No. 2,004,388.

As shown in FiG. 5, helical ns 21 have been mounted on the sheathing tube outer surface 22 by forming the fin grooves 213 and securing dus 21 therein in the conventional manner. Thus,'a iinned sheathed tubing construction is provided from the sheathed tubing construction of FiG. 4 in asimple and conventional manner.

lt is preferred, where iins such as the tins 21 are added to the sheathed tubing construction, that the combined generally radially` extending lengths of the sides 19 of the sheathing metal portions 1S in abutting contact with the side walls Zd of groove 17, as shown in FIG. 4, are

at least as great as the generally axial vor longitudinal widths or thicknesses of the tins-21 at the outer surface .22 of the sheathing tube 16, to provide an uninterrupted path of heat yflow from iins 21 into the inner basic tube 15. Furthermore, to make up this continuous uninterrupted path of heatfiiow between `the fins 21 andinner basic tube 15, it is likewise preferable to have the gen- Y eraliy circumferential Widths of thesheathing metal por* tions 1S at the mouths of the lgrooves 17 or at the outer circumferential surface of the inner basic tube 15 at least as great or greater than the axial `or longitudinal widths or `thicknesses of the fins r2.1 at `the outersurface 22 of the sheathing tube 16. Finally, it ispreferable to have` the total-lengths of the inner basicvtube grooves 17 at least as great as the total lengths of theiins 21 in order to have the total lengths of insured contact under any temperature conditions between the inner basic tube 15 and sheathing tube 16 at least as great as the total length of the iins 21, thereby again insuring a proper path of heat flow between the tins 21 and the inner tube 15f Various other forms of the sheathed tubing construction of the present invention may be made as desired and dependent on the particular final use of the tubing. For instance, as before described, the helical hns 21, as shown in FIG. -`5, may be replaced by other forms of tins, lfor instance, axiallyzfor longitudinally extending ii'ns, or tins merelyutension-Wrapped around-:the outer surface 22 of the sheathing tube 1o, all fofwhich/substitutions .are`

sheathed'tubing and for convenience in construction and fabrication as desired. Various other forms` of the grooves 17 on the basic tube 15 are illustrated for specific examples in FIGS. 9, l0 and ll.

In FIG.l 9, circumferentiallyspaced, staggered, axially or longitudinally extending grooves 24 are shown formed in the outer surface of thebasic tube 15. In FIG. 10, heiically extending grooves 25 are shown formed in the outer surface of the basic tube 15. Finally, in FIG. 11, both circumferentially spaced, straight, axially or longitudinally extending grooves 26 are shown in combination with axially or longitudinally spaced, circumferentially extending grooves 27.

Also, these combinations of circumferentially spaced axially or longitudinally extending grooves which are continuous or staggered, helical grooves, and axially or Vlongitudinally spaced, circumferentially extending grooves may be made' asi circumstances dictate. In every case, however, itis preferred to form the grooves generally dove-tail-in cross section, as previously described, with reference to the sheathed tubing construction of FIG. 4. IntFIGS. 12 and 13, a still further modification of the sheathed tubing construction is shown in combination withafheat exchanger tube sheet. In this case, the inner basicV tube 15 is telescoped with the sheathing tube 16 ndfcircunlferentially spaced, straight, axially Vor longitudinally extending grooves .23, preferably having dovetail cross sections, are .formed in the outer surface of the inner (basic tube, with the Isheathing tube being embedded V'therein to fornrthe interengagement or metalto-metal interlock betweenrthe inner basic tube 15 and sheathing tube 16. Furthermore, the grooves 2S terminate short of the tube sheet 29, with the portion of the inner basic tube 15 extending within the Vtube sheet being free of such grooves, although still having the sheathing tube 16 enca'sing and protecting and continuing into the tube sheet Z9. The purpose of discontinuing the grooves 28 short of the `tube sheet 29 is to eliminate paths for leakage of `tluid between the inner basic tube 15 and sheathing Vtube 16 through these grooves 2S, and it is impossible or sheathed nned tubing. 'lf'hisrparticularV embodimentV illustrates an internally sheathed tubing construction accordng to the principles ofthe present invention.

Referring to the sheathed tubing constructionk of FIG.

1,16, an .outer basic tube 115, preferably of steel for limit the principles of the present invention to the specific form of the grooves shown.

Thus, with the interengagement or metal-toLmetal interlock provided between the inner basic tube 11-5 and inner sheathing' tube 116, la secure physical interengagement is provided between these tubes which will maintain the tubes together during varying temperatures conditions despite the variations of the coeicients of thermal expansion of the materials forming the tubes. This, therefore, insures at all times proper paths of heat ow between the tubes providing an efficient tubing construction for heat exchanger construction use.

As discussed with reference tothe externally sheathed tubing ,construction of FIG. 4,' this internally sheathed tubing construction of FIG. 16 is formed with the general-ly radially extending sides 119' of'the sheathing metal portions 118 tightly abutting the generally radially exstrength, as previously discussed, is telescoped overthe relatively, thin inner sheathing tube 116, with the sheathing tube 1,16 again preferably being formed of the softer relatively non-corrosive material, such as aluminum or copper.

A preferably tight interference rit is provided between the outer basic tube `115 and the inner sheathing tube 1-16,

and this tight lit may be provided in numerous conven'- tional ways, such as, for instance, by expandingthe .inner sheathing tube'outwardlyI against the outer basic tube.V

Despite this tight rit, however, if the internallyv'sheathed i i tubing Vconstruc'ztion thus far described were used ina Y Vheat exchanger(constructinundenvarying,temperature conditions, the differences @of coecients of .thermal ex- For ,this reason, 1an-interengagementVA or metal-to-metal i interlock is provided preferably by the dovetail grooves 117 formed -in the innersurface of the basic tube 11,5 and Y into which have been formed the sheathing metal portions v 1138, formed integral with the remainder of the sheathing tube 116. In'this internally sheathed tubing construction,

-, itis preferred to form thedovetail grooves 117 extending generally axlially `orflongtiudinally of the tubes 115 and Y 11-6 and circumferentially spaced, asshown but it is also possible to provide the dove-tail grooves 117 extending, for instance, helically, if desired, and it is not intended to tending side walls 12.0' of the grooves 117, in order to.

insure the proper path Aof heat ow between the outer.

basic tube `115 and vinner sheathing tube 116 even though portions of the sheathing tube might separate from the outer basic tube-at llocations spaced between these grooves 117 under the varying temperature condi. tions. Thus, the broad principles involved in the construction of the externally sheathed'tubing of FIG. 4, as

. discussed above, are substantially the same as the'broad principles involved with the construction of the internally sheathed tubing of KFIG. 16, and it is obvious that sheathed tubing in which the basic tube is sheathed bothY internally and externally could be provided merely by a combination of the LFIG. 4 and FIG. 16 constructions.

r If it is desired to provide fins on this internally sheathed tubing constructoin, it is merely necessary to apply conventional ins to the outer surface of the outer basic tube 115 in any conventional manner to provide the internally sheathed linned tubing of FIG. 17 In this case, if the outer basic tube'were formed of steel, it would be preferable to form these outer fins or steel, since they are secured to tube 115 and would thereby have the same coecients of thermal expansion as this outer basic tube;

Referring to FIG. 17, the helical fins 121 could be mounted on the basic tubeV outer surface 122 by forming the fin grooves 123 and secuning the fins l121 therein in the conventional manner and as previously discussed with reference to the externally sheathed tubing construction having ns `formed thereon, as shown'in FIG. 5. Furthermore, the preferable comparative sizes of the various members of this internallysheathed tubing construction are the same as in the foregoing externally sheathed tubing construction and as previously discussed.

An embodiment of one method or the present invention is shown in FIGS. 6, 7 and 8 and may be related to' .the various stages of the externally sheathed tubing con- (of thesefsteps'inv and .of themselves being conventional,"

other 'than `in the particular order asset forth in the following. l 1 f r 6, a conventional groove cutter 30V `is` illustratedl forming the Vgenerally U-shapeld grooves 3:1 v i Vin the outer surfac'eg of lthe-'inner basic tube `15 -toprovide theconstructiori ofFIG.` .1, which is the Airststep in this As shown in PIG.

one form of the'niethod. The second stepl ofthe method is shown in F1017 in which a roll 32, through rolling pressure and by bridging'the VU-'shaped grooves 31, forms Y these U-shaped grooves'into thedove-tailgrooves 17, as

shownv in FIG. 7, and also in the second stage of the, construction, illustrated in FIG. 2t.'

Then, in this particular lform ofthe method,the.sheath` ing 'tube 16 Ais telescoped over the inner basic tube 15 resulting in the construction of FIG. 3, and this may be accomplished by providing a usual slip 't .between theY basic tube and sheathingftube. Finally, to complete the sheathed tubing construction, Vas shown in FIG. ifa` roll 313, extending substantially coaxially with the basic tube and sheathing tube 16, is used to roll progressively axially or longitudinally along the outer surface 22 of the sheathing tube 16 to decrease the diameter of the `sheathing tube 16 and form this sheathing tube tightly against the outer surface of the basic tube 15, While at the same time forming portions of the metal of the sheathing tube 16 radially inwardly into the dovetail grooves 17, thereby forming the interengagement or metal-to-metal interlock between the inner basic tube 15 and sheathing tube 16.

If a iinned sheathed tubing construction is desired, for instance, such as the construction of FIG. 5, it is then only necessary to take the sheathed tubing construction previously formed and apply the iins 21 in a conventional manner. 'Ilhese iins, as previously discussed, can be rformed elicaly, axially or longitudinally extending, secured in n grooves 23, as shown in FIG. 5, or merely wound on the outer surface 22 of the sheathing tube 16, as desired or as circumstances dictate. Also, the ins could be formed or extruded radially outwardly from their same metal and integral with the sheathing tube 16 in a conventional manner, and this is particularly easy :if the inaterial of the sheathing tube 16 is a softer metal, such as aluminum or copper.

The particular =forrn of the method shown in the drawings and described above is merely one form of method fior forming the product ofFIG. 3 or 4, and other dorms could be used lfor accomplishing the same general method steps as described. 1t is not intended, therefore, to limit the broader concept of the method of fthe present invention to the particular exact method steps shown.

The particular method steps illustrated and described above can be summarized as including the step of torming generally U-shaped grooves 31 in the outer circuferential surface of the inner basic tube 15, rolling of 4forming the U-shaped :grooves 31 into the dove-tail grooves 17, telescopinfg, the sheathing tube 16 over the inner basic tube x15, rolling or tiorming the sheathing tube 16 against i the outer circumferential surface of the basicVV tube 15 to provide a tight or interference tit therebetween, preferably at the same time rolling portions of lthe sheathing f tube 16 down into tight interengagement lor metal-to-metal interlock in the Idove-tail grooves 17 of the inner basic tube 15, and finally, Where iins are desired, `forming the fins 21 on the outerysurface 22 of the sheathing tube 15.

QAs" an illustration of the Vrolling *step of the `foregoing method, as shownin FIG. 8, in whichthe sheathing tube t 16V is'rolledtightly aigainstthe iouter'surface of the inner basic tube y15 and at the same timepo'rtio'ns of the sheathing tube metal are rolled intothe basic tube dovetail groove 17,V the original outer diameterfoff Vthe sheathing tube 16, which in the panticular 'case was formed of aluminum, was 1.000inch, and was telescopedV `with la slit lit over the inner basic tube 15.` After the `completion ofthe rolling operation, Aas shown in HG.` 8, the outer diameter orf the `aluminum sheathing tube in this parf` ticular case was 0.9S5inch, "had-a-tightiit with the inner basic tube 15, and was tightly, interengaged or iriterlocked` into the basic tube dove-tailgroove17."`

lIf 'it `is desired, when the-inner basic tubes" `115-tare originally fabricated Aat the mills, ,the` dove-tail Vgrooves 17 maybe directly 'extruded in` he outer surface ofthe s 10 summarized as iextinding 1or forming the dove-tail :grooves 17 in the outer surface of the inner basic tube 15, applying or telescoping or extruding the sheathing tube '16 over the inner basic tube 15 and forming the sheathing tube 16 into the basic tube dove-tail grooves 17 to provide the nietal-to-metal interlock, and, =where desired, applying uns 21 to the outer surface 22 of sheathing tube 16 either within the tin grooves 23 or otherwise. "The various stages of the construction. of internally sheathed tubing are shown in FIGS. 14, 15, 16 and 17, and conventional metal working operations may be used lfor forming this construction. The outer basic tube, as shown in VFIG. 14, may be lformed with the dove-tail cross-section .grooves -117 in the inner surface thereof by originallyextnuding these basic tubes with the grooves therein in a mill operation, or by hrst forming the bas-ic tubes and then removing the metal to form the grooves by usual means, such as by a conventional spider-type cutting tool. As before stated, these grooves may be formed extending in various paths as desired and as conditions demand -for securing the unal physical interengagenrent or metal-to-metal interlock .discussed above.

As shown in FIG. 15, the inner sheathing tube 116 is then telescoped within the outer basic tube 115. This may be done by providing a slip tit between the two tubes or by originally extrudin'gthe inner sheathing tube 116 within the outer basic tube 115.

Finally, portions of the inner sheathing tube 116 are formed into lthe preferably dove-tail cross-section grooves 117 of the outer basic tube 115. If a slip tit is provided between tubes 115 and 1-16 for the original telescopic assembly, the inner sheathing tube 116 is then expanded outwardly into preferably a tight interference iit with the outer basic tube 115, and at the same time the sheathling metal portions 11S are expanded into tight physical interengagenient in the dove-tail cross-section Igrooves This expanding of the inner sheathing tube 116 may be done by Vmany `conventional processes, such as by ball expanding, drawing pressurizin-gfor by applying a shock wave. It the inner sheathing tube 116 is originally extruded int-o the outer basic tube 115, then at the same time the sheathing metal-portions 1 18-may be ext'nuded and formed into the dove-tail cross-section grooves 117, andthis may be done despite the directionV thatfthese grooves 1417 extend, by the proper application of pressure during this extrusion process.

'It Finally, after the internally sheathed tubing construcy tion is `formed, as shown in FIG. 16, by one of the foregoing processes or methods, conventional forms ont fins 121 may be applied if and as desired in a conventional manner, suchas secured Vwithin the iin :grooves 123 .formed inwtheiouterV basic tube 115 and as shown in FIG. 17. Other forms of fins, of course, could be provided dependent fon therequirements of the particular use of the construction., Y i

` Thus, in this method for forming the internally sheathed tubing construction of FIG.'A 1'6`or` 17, the method steps Vcan!` be summarized as extruding or` otherwise? -forming therpreferably' idovetail'crosssection `grooves 117 in the innersurfacerof the router basic tube 115,te1escoping l' ort extruding the sheathing tube 116 Iwithin `the outer thereon.; Thereafter, the sheathin tube 16` can then be extruded' directly over theouter-'ts'iirface `of thetinner basic'V ytube 1.5-, the portions ofthe sheathing Vtube 16 beingextnuded directly into the :d'nif'e-tai'll grooves `1'7 of the basic tube -15 to provide fthe` interengagernent or metal-to-.metal interlock therebetween. `Furthermore,`

this texitnnding ofA the sheathing tube 16 over the inner basic `tube 15andv into the basic tube dove-tail groove 17 can be accomplished regardless of whether the dove-tail grooves extend straight axially or longitudinally, helically,

directlylcircumfe-rentially, or any .other `forms comprised t basic tube 115, andV forming` the sheathing tube 116 into the ba'sictube dove-tail groovesfllTto pnovide the metalfte-metal interlock Where desired, the method Vcanfurther t inclurdethe applying :of the tins 121 to the outer-surface 122.of the outer basic tube either within the fins rgrooves'123` orotherwise. t t

i "A still turther `method Vfor forming the externally Vsheathed `:tubing construction of FIG. 4 or the internally sheathed tubing construction of FIG. 16 may be the combining of a coating step 'for forming a sheathing tube telescoped either externally or internally with the basic tube, `and preferably then in some usual manner, such asl ext-nuding or other metal-working operations, forming 1 14 a relatively smooth exposed surface on the sheathing tube. In this case, `the embedding or interengaging of the sheathing tube metal in the basic tube grooves would be accomplished, at least principally, during the coating step.

For instance, considering the externally sheathed tubing construction of FIG. 4, preferably the dovetail cross-sec tion grooves 117 would be first formed in the outer'surface of the inner basic tube 15. This inner basic tube could vthen be passed through a coating pot containing molten metal of the type Iforming the outer sheathing tube 16, to provide a complete coating of basic tube 15 with the `metal of the sheathing tube A16 and thereby `form the inner basic tube telescoped with the outer sheathing tube, while at the same time the metal of the sheathing tube 16 would form properly Iinto the basic tube dove-tail grooves 17 v[providing the physical interengagement -or metal-.tom-et-al interlock bet-Ween the tubes.

After the complete coating of the outer surface of the basic tube 15 is completed to form the sheathing tube 16 telescoped thereover, the telescoped tubes could then be passed through an extruding or simil-ar die for forming the sheathing tube of substantially uniform thickness over the basic tube, or this operation could be accomplished by other usual metal-working operations, such as rolling .and thelike. lFinally, if desired, the fins 21 couldV be formed on the iouter surface 22 ofthe sheath-ing tube 16 to provide the construction .of FIG. 5.

This same general method could be used to provide the y internally sheathed ltubing construction of FIG. 16, that is, Yforming the preferably doven-ail cross-section grooves .117in the inner surface of the outer basic-tube 115, substantially completely coating this basic tube inner surface with the metal of the sheathing tubeV 116 to `form the outer basic tube internally telescoped with the inner sheathingtu-be, 'and at the same time forming the metal of the sheathing tube 116 outwardly into and physically interengaged with the basic tube grooves 117, .and then preferably -extruding or otherwise forming the metal of the sheathing tube of substantially uniform thickness. Again, Where'fdesired, the tins 121 can be applied to the outer surface 122 of the outer basic tube 115 to form the internally sheathed and iinnedtubing construction of 1116,17.v This further alterna-te method for forming the `exter- ,nally or internally sheathed tubingrconstructions of FIG.

4 or 16 can, therefore, be summarized as formi-ng grooves in ran external or .internal surface of aibasic tube 1&5 or 1:15, substantially completely coating this basic tube surl Ytace to formv'a sheathing tube V16' or 116 telescoped ex-Y ternallyior internally with thevbasic tube 15 or 115 and alsoforming the sheathing tube :metal into the basic tube i grooves17 or '117, .and then preferably extrudingorcther the finnedssheathed tubing,constructionsiof,FIG. 5'crf'17,

it wouldbemerelynecessary to Vadd the step of applying y Ythe Liins f2.1 yor 121 .tothe outer surface of the sheathing st ube'ldror the `louter surfaceofthe basictube 115.

g Thus, according to the methods of the .presentinvenf tion, `:an externally orl .internally sheathed tubing construe, tion `or -linnedsheathed-tubing construction is provided in which thebasic tube 15 or 115 may be formed of materials, suchy .as steel,-A to providev the required. strength 'for high-pressure applications, andthisjbasictube' is completely externally or internally sheathed-and protected trom externalor internal corrosive fluids by .the sheathingtube Y16 '011116 lformed preferably of a softer'rnetal 1 not readily. subject-to corrosion. Further, 'whetherexternal or internal, the sheathing tube 16 or 116 is interengag'ed vwith the basic tube 15 or 115 by va metal-toametal interen'gagement or physical interlock so that even though the'mate-rials forming the basic tube 15 or V115 and sheathing tube 16 .or-'116 have different coefficients of thermal expansion, the sheathing tube 16 or 116', due to the met-alto-metal interengagement cr physical interlock, remains in proper heattransfer contact or heat flow contact with the basic tube 115, so that the construction is highly efficient in Varying temperature vvork.

It is preferred, in Iforming .the .metal-to-metal interengagement or physical interlock between the tubes according lto .the constructions :and methods of the present invention, to xform Ithe preferably dove-tail cross-section grow-esV 17 or 117 in the surface of .the tube formed of the harder metal andro lform the softer metal of .the other tube into .these grooves. Thus, as shown and described, it is preferred to for-rn the grooves in the steel basic tube 15 or 115 .and to 'form fthe metal of the softer aluminum or copper sheathing tube 16 cr 1-16 into these grooves, but Where the metals "of the two tubes involvedare lof sub# stan-tially the samehardness .and streng-th, the grooves 17 or 117 .could be .formed in either tube, with .the metal of the other tube formed therein.

Finallyythe externally or internally sheathed `tubing con struction, according to the principles of the present invention, may be formed ina relatively simple manner by conventio-nal metal working operations .and by any one-of a number of relatively simple and foolproof methods, Vand fins may be added to this tubing construction as desired, also according to conventional iin-.applying methods. This, therefore, provid-es the iinned or unnned, externally or internally, sheathed tubing construction at a relatively low cost. Y i

In the .foregoing description, certain term-s have been used for brevity, clearness and understanding, but no unf necessary limitations are to be implied therefrom, because such words are used rfor descriptive purposes herein and s are intended to be broadly construed.

Moreover, the embodiments of the improved construction and methods illustrated and :described'herein are by Way of example, and lthe scope of the presen-t invention is n-ot limited to the exact ldetails of the construction vand methods shownand described.

Having nowdescribed the invention, the construction, operation 4and use of preferred embodiments thereof, the methods and procedural steps `for forming .such construe# tions, and the"advantageous new and useful results lobtained thereby; the 4new and useful constructions and meth'- odsrand equivalents thereof obvious tothose skilledlin the art are set 4forth in the appended claims.

.Weclaim: 1; Y 1

v1. `The method of forming sheathed tubing including the steps' of forming groove means in a surface of .a first metal tube with said groove means extending inwardly or' thevtube metal from said tube surface and opening outwardly at said tubesunface, extrudiug a substantially con;

tinuous vmetalsheathing tubeV telescoped with `the Viirst tube surface with said ,sheathingl tube tightly abutting the iirst tube surface,- and during the extruding of the sheathing tube-` telescopedwithV the first tube surface Yforming and embedding portions. of the sheathing tube inwardly intoV and interlockingy the'lsheathing metallwith the iirst tube groove` in eans therebyprovidin'g -aninterj locking. metal-toametalheat"transfer bond betweenV said 'tubes' for vz rnaintaining. the tubes intintirnate heat transfer ontactunder varying temperature conditions. V

2.,*'1`he method of; formingl sheathed tubingv including the steps of forming groove means .in a surface, of aiiirst metal tube with said. groove means ,extending inwardly ofrthertube rnetal'jfrom'4 said tube ,surface and opening outwardly atV said` tube surface,.extruding'a substantially continuous metal sheathingjtube telescoped with the iirst tulbe surface with said, sheathing tube tightlypabutting" the rst tube surface, during the extruding ofthe sheath-i ing tube telescoped with the iirst tube surface forming and embedding vportions of the. sheathing tubefinwardly mto and interlocking the sheathing metal with the first tube thereby Vproviding an interlocking metal-tometal heat transfer bond between said tubes for maintaining the tubes in intimate heat transfer contact under varying temperature conditions, and forming hn means on the outer surface of the outermost of said tubes in inti* mate heat transfer contact and extending generally radially from the louter surface of said outermost tube.

3. The method of forming sheathed tubing including the steps of forming groove means in the outer surface of a first metal tube with said groove means extending inwardly off the tube metal from `said rst tube surface and opening youtwardly at said tirst tube surface, extruding a substantially continuous metal sheathing tube telescoped over the Afirst tube louter surface with said sheathing tube tightly abutting the rst tube outer surface, during the extruding of the sheathing tube telescoped over the rlirst tube outer surface forming and embedding portions of the Asheathing tube inwardly into and inter locking the sheathing metal with the first tube 'thereby providing an interlocking metalJtO-rnetal heat transfer bond between said tubes for maintaining the tubes in intimate heat transfer contact under varying temperature conditions, and forming fin means `on the outer surface of the sheathing tube in intimate heat transfer contact and extending generally radially from the louter surface of said sheathing tube.

4. The method of forming sheathed tubing including the steps of extruding groove means in a surface of a first metal tube with said groove means extending inwardly of the tube metal fnom said tube surface and opening outwardly at said tube surface, extruding a substantially continuous metal sheathing tube telesooped With 14 fa'ce forming and embedding portions of the sheathing tube inwardly into and interlocking the sheathing metal with the first tube groove means thereby providing an interlooking metal-to-metal heat transfer bond *between said tubes for maintaining the tubes inlintimate heat transfer contact under varying temperature conditions.

5. The method of forming sheathed tubing including the steps of extruding groove means in a surface of a first -rnetal tube with said groovemeans extending inwardly of the tube metal from said tube surface and opening outwardly at said tube surface, extruding a substantially continuous metal sheathing tube telesooped with the iirst tube surface with said sheathing tube tightly albutting the first tube surface, during the extruding of the sheathing tube telescoped with the rst tube surface forming and embedding portions of the shea-thing tube inwardly into and interlocking the sheathing metal with the iirst tube groove means thereby providing an interlocking metaltometal heat transfer bond `between said ,tubes for maintaining the tubes in intimate heat transfer contact under Varying temperature loonditions, forming fin Agroove means in the outer surface :of the outermost tube, and securing fin means in said iin groove means in intimate heat transfer contact with and` extending generally radially from said outermost tube.

References Cited in the file of this patent UNiTED STATES PATENTS 309,439 Chillingworth ...1 Dec. 16, 1884 2,050,993 Bush Aug. 11, 1936 2,754,577 MaXWell `uly 17, 1956 FOREIGN PATENTS 881,486 France Jan. 2-8, 1943 

1. THE METHOD OF FORMING SHEATHED TUBING INCLUDING THE STEPS OF FORMING GROOVE MEANS IN A SURFACE OF A FIRST METAL TUBE WITH SAID GROOVE MEANS EXTENDING INWARDLY OF THE TUBE METAL FROM SAID TUBE SURFACE AND OPENING OUTWARDLY AT SAID TUBE SURFACE, EXTRUDING A SUBSTANTIALLY CONTINUOUS METAL SHEATHING TUBE TELESCOPED WITH THE FIRST TUBE SURFACE WITH SAID SHEATHING TUBE TIGHTLY ABUTTING THE FIRST TUBE SURFACE, AND DURING THE EXTRUDING OF THE SHEATHING TUBE TELESCOPED WITH THE FIRST TUBE SURFACE FORMING AND EMBEDDING PORTIONS OF THE SHEATHING TUBE INWARDLY INTO AND INTERLOCKING THE SHEATHING METAL WITH THE FIRST TUBE GROOVE MEANS THEREBY PROVIDING AN INTERLOCKING METAL-TO-METAL HEAT TRANSFER BOND BETWEEN SAID TUBES FOR MAINTAINING THE TUBES IN INTIMATE HEAT TRANSFER CONTACT UNDER VARYING TEMPERATURE CONDITIONS. 